1. Field of the Invention
The present invention relates to an image fixing apparatus (device) adapted for use in an image forming apparatus such as a copying apparatus or a printer, and more particularly to an image fixing device of a low heat capacity.
2. Related Background Art
Conventionally, the electrophotographic image forming apparatus such as copying apparatus or printer is provided with a fixing device for fixing an unfixed image (toner image) formed on a recording material.
A part of such fixing devices employs, as the fixing means, the film heating type of energy saving type.
The fixing device of the film heating type is disclosed for example in the Japanese Patent Laid-open Application Nos. 63-313182, 2-157878, 4-44075 to 4-44083, and 4-204980 to 4-204984.
In the image fixing device of the film heating method, a heat-resistant film (fixing film), constituting of a rotary heating member, is made to slide in contact with a heating member by a pressing rotary member (elastic roller), and a recording material bearing an unfixed image is introduced into the nip portion formed by the heating member and the pressing member across the heat-resistant film and is conveyed together with the heat-resistant film, whereby the unfixed image is fixed as a permanent image onto the recording material by the heat supplied from the heating member through the heat-resistant film and the pressure in the nip portion.
Such fixing device of the film heating method can achieve saving of the electric power consumption and reduction in the waiting time (improvement in the quick starting performance), because the heating member can be composed of a linear heating member of a low heat capacity and the heat-resistant film can be composed of a thin film of a low heat capacity.
In such fixing device, there is known a type in which a driving roller is provided inside the internal periphery of the film and the film is driven under tension, and another type (pressing rotary member driving type) in which the film is loosely fitted on a film guide and is driven by the pressing rotary member.
Recently the latter type (pressing rotary member driving type) is widely employed because of the reduced number of components in this type.
In the fixing device of the film heating method explained above, the heat capacity has to be reduced for realizing the on-demand performance, and there is required temperature control as explained in the following.
In the fixing device of the film heating method, since the amount of heat given to the paper varies considerably by the temperature of the pressing roller, it is necessary to vary the controlled temperature so as to give a constant amount of heat to the paper, depending on the number of prints or the time elapsed after the preceding fixing operation.
More specifically, the controlled temperature is set higher when the temperature of the pressure roller is low after the start of power supply, but is gradually lowered when the temperature of the pressure roller is elevated by the repeated printing operations.
Such temperature control allows to prevent the defective image fixation or the hot offset phenomenon.
FIG. 6 shows an example of the temperature control, wherein the abscissa indicates the number of prints while the ordinate indicates the controlled temperature.
The example shown in FIG. 6 is the case of continuous printing, in which the controlled temperature is lowered at every 10 sheets.
The controlled temperature is however not lowered endlessly. As the temperature of the pressure roller becomes stabilized after processing about 50 sheets, such stabilized temperature is selected as the final controlled temperature.
Also in case of the intermittent printing, in which printing operation and pause alternate, the interval between the printing operations or the internal between the sheets becomes larger, so that the pressure roller is heated for long time by the heater through the fixing film and the temperature of the pressure roller becomes higher than in the continuous printing operation.
Consequently the controlled temperature is lowered from the fewer number of sheets than in the continuous printing operation.
It is indicated by a broken line in FIG. 6.
However, in case of the intermittent printing, temperature control cannot always be made same, because the temperature of the pressure roller varies depending on the length of pause between the printing operations.
For example, in the conventional method, the last temperature control is executed after intermittent printing of 30 sheets, but, if a long pause period follows thereafter, the high controlled temperature is adopted again because the temperature of the pressure roller is lowered.
For this purpose, in order to estimate the temperature of the pressure roller, the temperature of the heater has been detected with a sensor such as the thermistor provided on the heater for temperature control and is entered into a corresponding table for realizing optimum temperature control.
However, in the above-described on-demand fixing device, there has only be considered the temperature of the pressure roller in a portion corresponding to the thermistor.
Consequently, after the printing of a small-sized sheet such as an envelope, the temperature of the pressure roller can be detected in the area passed by such sheet in the longitudinal direction of the pressure roller, but, since the heater generates heat uniformly over the entire sheet passing area in the longitudinal direction of the pressure roller, the temperature becomes higher in the area not passed by the sheet because the heat is not carried away by the sheet (so-called temperature elevation in the sheet non-passing area).
In such situation, if the heater temperature is controlled by the temperature detected by the thermistor present in the sheet-passing area, the image fixation becomes excessive in the sheet non-passing area, resulting in image smear by the hot offsetting.
Also in case a small-sized sheet is passed in the nip portion whereby the temperature of the pressure roller becomes higher in the sheet non-passing area and a next sheet is larger than the preceding sheet, the moisture contained in such sheet evaporates, at the entry into the fixing nip, by the heat of the high temperature portion of the pressure roller and is deposited on the pressure roller, particularly in the portion of the non-higher temperature.
It is revealed that the moisture deposited on the pressure roller reduces the transporting ability thereof, whereby the sheet causes slippage and becomes unable to enter the fixing nip.
It is also found that such phenomenon is apt to occur in case of using the pressing roller drive type or in case of using the pressure roller of good releasing property (such as a roller of which surface layer is composed of a fluorinated resin tube).